Electrical insulating oils



United States Patent Otli ce 3,044,955 ELECTRICAL INSULATING OILSWilhelmns C. de Groot and Hubertus N. H. de Haan, both of Amsterdam,Netherlands, and Ralph W. van den Bosch, Sevenoaks, England, assignorsto Shell Oil Company, a corporation of Delaware No Drawing. Filed May27, 1959, Ser. No. 816,073 Claims priority, application Great BritainJune 3, 1958 1 Claim. (Cl. 208-212) This invention relates to improvedinsulating oils. More particularly, it relates to a process forpreparing electrical insulating oils which are oxidation stable,nonsludging, and resistant to gassing.

Certain hydrocarbon oils are used in electrical applications such astransformers, circuit breakers, and high voltage cables because of theirhigh dielectric strength. However, because of the susceptibility of suchhydrocarbons to oxidation, which results in the formation of sludge andacid, hydrocarbon oils for electrical applications must be carefullyrefined. The high electrical stress to which insulating oils aresubjected also tends to decompose the oil thus producing sludge andgaseous products which may cause serious damage and failure of theapparatus in which they are used. The presence of aromatic hydrocarbonsin the oil tends to inhibit such decomposition (gassing), but they havean adverse effect in that they increase oxidative sludging. Theprincipal detrimental effect of sludging and acid formation and gassingis to raise the dielectric constant and power factor. In addition,substantial corrosion of electrical components is often caused by theacids formed by sludging and gassing. Drastic refining of highlyaromatic oils to produce more stable electrical insulating oils resultsin poor yields, which are generally as low as 45-55% by weight,calculated on the basis of the starting material.

Mineral oils having a viscosity of between about 1 and 9 degrees Englerat 20 C. (generally between 3 and 5 degrees Engler at 20 C.) aregenerally used as dielectric oils in electrical equipment.

For preparing electrical insulating oils, the starting material isusually a straight-run mineral oil distillate of the above viscosity.Usual refining methods are extraction with a selective solvent for thearomatic components of the oil (e.g., liquid sulfur dioxide or furfural)and treatment of the rafiinate with concentrated sulfuric acid or oleum.Combinations of these treatments may be used and they are usuallyfollowed by a finishing treatment with a solid adsorbent, such as clay.If desired, the oils thus obtained may also be mixed with a small amountof an extract obtained by selective extraction after this has also beentreated with concentrated sulfuric acid or oleum. In this connectionreference may be made to British specifications 589,149 and 589,150.These usual refining methods have the disadvantages that they are rathercostly and that considerable amounts of the oil are lost, as mentionedabove.

Desulfurization by selective hydrogenation (frequently termedhydrofining or hydrodesulfurization) is a refining method which is cheapand wherein only small amounts of the oil are lost. It has already beenproposed to apply this refining method in the preparation of electricalinsulating oils. From Brennstofichemie, 36, 218 (1955), it is known toprepare an electrical insulating oil by first removing the sulfurcompletely from a shale oil by hydrogenation and then treating thehydrogenated oil with sulfuric acid and bleaching earth. From The Oiland Gas Journal, 53, 84 (1954) it is known to prepare an elec tricalinsulating oil by extracting a distillate obtained from a Tia Juanacrude oil with liquid sulfur dioxide, so as to obtain a raifinate havinga sulfur content of 0.42% by weight and then hydrogenating the raflinateso as to obtain an oil having a sulfur content of 0.06% by weight. Thus,in these known processes the hydrogenation is still combined with anextraction or an acid treatment.

It is therefore an object of this invention to provide a process for theproduction of electrical insulating oils with satisfactory properties ina simple and inexpensive manner and in high yields. It is a furtherobject of this invention to provide a process for the production ofinsulating oils having excellent oxidation stability and very lowgassing tendencies. Other objects will be apparent from the detaileddescription of the process of this invention which comprises aparticular combination of selective hydrodesulfurization of mineral oildistillate and treatment with a solid adsorbent.

By selective hydrodesulfurization is here meant a hydrogenation underconditions in which the non-hydrocarbons present in the oil (viz. thecompounds which in addition to carbon and hydrogen contain otherelements, such as sulfur or oxygen) are hydrogenated, but the hydrocarbons remain essentially unaffected (so that, for example, there isvirtually no cracking of the oil or hydrogenation of aromatics presentin the oil). Several methods are already known (see, for example, Erdoland Kohle, 6, 616-619 (1953)) for selectively hydrogenating oils in thisway. The oil may be in either the gas or the liquid phase during thehydrogenation, a very suitable method being trickle phase hydrogenation,wherein a very thin layer of the oil is allowed to flow over a bed of acatalyst. In general the selective hydrogenation is carried out at atemperature of 300-400 C., a pressure of 10-200 kg. per sq. cm., a gasdischarge rate of 50- 5,000 liters per kg. of oil and a flow rate of theoil of 0.3-3 kg. per liter of catalyst per hour. Suitable catalysts areoxygen or sulfur containing compounds such as oxides and sulfides ofmetals of groups VI and VIII of the periodic table. Especially preferredare cobalt oxide, molybdenum oxide, ,tungsten sulfide and nickelsulfide. The catalysts are preferably such compounds supported on acarrier, such as active carbon, fullers earth, kieselguhr, silica oralumina, e.g. in the form of bauxite, pumice or burnt clay. Verysuitable catalysts are described in British specification 657,400 and ofthese the catalysts comprising cobalt oxide and molybdenum oxide on analuminum oxide-containing carrier, such as bauxite, are particularlypreferred. Of the latter class of catalysts, the catalyst referred to ashigh cobalt cata-1. lysts which contain cobalt and molybdenum in an atomic ratio between about 0.7 and 0.8, are particularly eifective. Thecatalysts may be used in the form of cylindrical tablets measuring, forexample, 5 x 5 or 3 x 3 mm.

Suitable hydrogenation methods are described for example in Britishspecifications Nos. 657,521, 665,575 and 699,455. r

An important feature of the invention is that the selectivehydrodesulfurization should be carried out so that, the oil is notcompletely desulfurized. Generally, the hydrogenated oil should stillcontain at least 0.05% by; weight of sulfur and in most cases not morethan of the sulfur originally present in the oil should be removed. Onthe other hand, the sulfur content of thehydrogenated oil should not betoo high and should genunexpectedly to the stability of the insulatingoils is not fully understood. The sulfur compounds remaining in v. theoil after the hydrotreating step are, of course, more Patented July 17,1962 i hydrogenation resistant and of different average composition andconfiguration than those which are removed. Therefore, because of thepartial removal of other sulfur compounds, and because of the differentcomposition of the residual sulfur compounds, they appear to exhibit aUnique inhibitory effect which is not realized from an oil of likesulfur content which has not been hydrodesulfurized or which has beendesulfurized by other means.

Within the above limits, the optimum extent of desulfurization dependson the specificationwhich the finished oil must satisfy. In thisconnection it should be noted that various methods for testing theoxidation stability of electrical insulating oils exist. However, theproperties of the oils produced according to the process of thisinvention as described herein are evaluated by the specification andtests of the British Standards Institution (BS1). Details of the BSItest, as it is hereinafter referred to, are found in test B.S. 148:1951of the British Standards Institution.

When desulfurized to the extent indicated above, an oil will be obtainedwhich satisfies at least one of these specifications and can, therefore,be considered to be suitable for use as an electrical insulating oil.Hydrogenation to a sulfur content of OAS-0.35% by weight (still morepreferably .200.30% by weight) is very suitable, since then from variousstarting materials an insulating oil can be obtained which satisfiesmore than one specification at the same time, which is of advantage tothe manufacturer from the point of view of economy of production.

In the selective hydrogenation, hydrogen sulfide is formed from thesulfur compounds present in the oil. A part of the hydrogen sulfideformed dissolves in the oil, and this should be removed from the oil.This can be effected, for example, by washing the oil with a causticalkali solution, but it is most simply carried out by blowing throughthe oil an inert gas such as nitrogen, if desired, at elevatedtemperature or by stripping with steam.

Electrical insulating oils are often required to have a flash pointwhich is not below a certain temperature. As a result of the selectivehydrogenation the initial boiling point of the oil is somewhat reduced.This causes a lowering of the flash point of the oil. As a result theflash point of the final insulating oil may not satisfy the flash pointrequirement. If this is the case, the low boiling components should beremoved from the oil. This can be done in a simple way by means ofsteam-stripping or distillation, preferably under reduced pressure,until the residue has the desired flash point. A Pensky-Martens closedcup (PMCC) flash point of at least 120 C. is desirable, and a PMCC flashpoint of at least 146 C. is preferred.

Finally, in the process of the invention, the oil is treated with asolid adsorbent such as fullers earths and activated clays. Suitableadsorbents, are, for example, fullers earths of the attapulgite andmontmorillonitic types and acid activated clays such as bentonite,bauxite, and alumina. Small quantities of alkaline material such as limemay be added to the adsorbent. If desired, the oil may be heated duringthe treatment (for example, to a temperature of about 50-100 C.), and aninert gas such as nitrogen passed through the oil. The quantity ofadsorbent used generally lies between 0.5 and 30% by weight, preferablybetween 1 and 5% by weight, calculated on the oil treated.

The process of the invention is generally applicable to mineral oilfractions having a viscosity between about 1 and 9 degrees Engler at 20C., i.e. light lubricating oil distillates, such as spindle oils. Inmost cases a distillate boiling in the range of from 300 to 500 C. isused as the starting material. Of course, the starting material shouldbe so selected, that after desulfurization to the prescribed extent thedesired sulfur content can be attained. The process of the invention isof particular advantage for starting materials having a relatively highsulfur content, ie oils having a sulfur content of at least 1% byweight, and particularly those having a sulfur content of at least 1.5%by weight.

It is not necessary to separate first the desired light lubricating oildistillate from a mineral oil and then to hydrogenate the separateddistillate and to treat it with a solid adsorbent. It is also possibleto start from a fraction which in addition to the desired lightlubricating oil contains heavier components, such as heavier lubricatingoils, and/ or lighter components, such as gasoline, kerosene or gas oil.Such a fraction may be selectively hydrogenated and the desired lightlubricating oil separated from it after the hydrogenation bydistillation. In this distillation the flash point of the separatedlight lubricating oil distillate fraction may be adjusted at the sametime to the desired value.

The aromatic content of the oil is reduced very little during theselective hydrogenation for the reasons stated above. Also during thetreatment with a solid absorbent no substantial reduction of thearomatic content of the oil occurs. A high aromatic content of anelectrical insulating oil has a favorable effect on its gassingcharacteristics, so that the process of the invention provides anexcellent means for preparing insulating oils with good characteristics.This becomes particularly apparent when starting materials having arelatively high aromatic content are used, e.g. oils having an aromaticcontent of at least 30% by weight and particularly those having anaromatic content of at least 40% by weight.

When, in this specification, reference is made to an aromatic content ofan oil, the aromatic content is that determined by percolation oversilica gel as described in Journal of the Institute of Petroleum, 36,89-104 1950).

The following examples illustrate the process of the invention.

EXAMPLE I Run A The starting material was a spindle oil obtained bydistilling a naphthenic crude oil. This distillate had the followingproperties:

Viscosity 5.5 degrees Engler at 20 C. Flash point (Pensky-Martens,closed cup) 156 C. Sulfur content 1.76% by weight. Aromatic content 50%by Weight.

Temperature, C 375 Pressure, kg./sq. cm 50 Flow rate of the oil, kg./liter catalyst/hour 1.9 Gas discharge rate, liter kg. oil 200Approximately 2% by weight of the oil was lost during the selectivehydrogenation.

The oil was stripped with steam in order to remove the hydrogen sulfideand the lightest components. Approximately 8% by weight of the oil wasremoved during this treatment. The remaining oil had a flash point of148 C., a sulfur content of 0.25% by weight and an aromatic content of45% by weight.

Finally the oil was treated at a temperature of 75 C. with 3% by weightof Filtrol for /2 hour while nitrogen was passed through it. The yieldof final electrical insulating oil was 88% by weight, calculated on thetotal amount of starting material.

The oxidation stability of the final oil was determined in the B.S.I.test (B.S. 148/1951). According to the content of 0.48% by weight.

lectively hydrogenated as described in Example I, with specification,after the conclusion of the test the limit of sludge formation in theoil should be 1.2% by weight and the maximum acid number of the oil 2.5.The oil satisfied this specification: after the test the oil had asludge value of 1.1% by weight and an acid number of 0.8.

The gassing characteristics of the final oil were evaluated in themodified Pirelli apparatus (Journal of the Institute of Petroleum, 35,No. 311, pp. 735-754) at a temperature of 50 C. under a hydrogenatmosphere and a voltage of 10 kv. The amount of hydrogen is measuredwhich the oil absorbs or evolves under these conditions in the course oftime. The hydrogen absorption or hydrogen evolution expressed in mm. ofoil pressure is plotted in a graph as a function of the time and theslope of the line thus obtained determines the gassing coefficient ofthe oil. For the present oil this coefficient was found to have the veryfavorable value of 8, which means that the oil is very strongly gasabsorbing.

In the following runs some comparative tests are described.

Run B The procedure of Run A was repeated, except that the Filtroltreatment of the oil was omitted. In the 13.8.1. test the oil had asludge value of 2.4% by Weight and an acid number of 2.0.

Run C The same starting material used in Run A was bydrogenated andstripped with steam as described for Run A. Then the oil was treatedwith 2% by weight of 96% sulfuric acid for /2 hour at a temperature of'20" C. The acid sludge was then allowed to deposit and was separated011?. Finally the oil was treated with 3% by weight of Filtrol, to which0.2% by Weight of lime had been added (the percentages are calculated onthe quantity of oil treated), at 75 C. for A2 hour while nitrogen waspassed through it. In the B.S.I. test the final oil had a sludge valueof 1.63% by weight and an acid number of 1.3.

Run D An electrical insulating oil satisfying the E81. test was preparedby a conventional method from the same starting material as was used inRun A.

The oil was first extracted with liquid sulfur dioxide (ratio by volumeof voil:SO =1:1.3. The raflinate thus obtained was treated with 10% byweight of oleum (containing 17% by weight of dissolved S for /2 hour ata temperature of 20 C. The acid sludge was then allowed to deposit andwas separated off. The oil was washed with a 4% solution of NaOH in 40%aqueous alcohol and subsequently with dilute alcohol and water. Finallythe oil was treated at 75 C. with 3% by weight of Filtrol for /2 hourwhile nitrogen was passed through it. The yield of final electricalinsulating oil was 55% by weight. In the B.S.I. test the oil had asludge value of 0.84% by weight and an acid number of 1.4. The gassingcoeificient of the oil was positive.

Run E The same starting material as used in Run A was extracted withliquid sulfur dioxide (ratio by volume of 0il:SO =l:2) so as to obtain aralfinate having a sulfur The rafiinate was then sethe exception thatthe reaction conditions were so modified that the oil (aftersteam-stripping) had a sulfur content of 0.04% by weight. The oil wasfinished by treatment with Filtrol as described in Example I. In the13.5.1. test the oil had a sludge value of 1.65% by weight and an acidnumber of 2.7.

The oxidation stability of oils prepared according to the process of theinvention (to be referred to hereinafter as oils of type A) can beimproved by the addition of mineral oil fractions having a loweraromatic c'on- B. It is clear that this possibility is particularly ofim-' portance for oils of type A having a high aromatic content. Ofcourse, the sulfur content of the oil of type B should not be such thatthe sulfur content of the final mixture becomes unduly high. Generallythe sulfur content of the final mixture should not be above 0.6% byweight.

The oil of type B is preferably obtained from the same starting materialas the oil of type A to result in an espe cially suitable electricalinsulating oil in accordance with the invention.

Oils of type B may be obtained by extracting an oil in a known mannerwith a selective solvent for the aromatic components of the oil, e.g.liquid sulfur dioxide or furfural. The oil of type B may also besubjected to other refining treatments, such as treatment withconcentrated sulfuric 'acid or oleum or selective hydrogenation.Generally the oil of type B is also subjected to a finishing treatmentwith a solid adsorbent. This finishing treatment may be carried out inthe same way as described above in connection with the preparation ofthe oil of type A. The finishing treatment may be carried out either onthe oils of type A and B separately, or after blending.

If during the preparation of the oil of type B extraction withconcentrated sulfuric acid or oleum is applied, this treatment may becarried out in any known manner. For instance, concentrated sulfuricacid (80100%) or oleum (up to 30% by weight dissolved S0 may be used inamounts of from 5-30% by weight, calculated on the oil treated, astemperatures varying from 0 to C. and contact times from 5 sec. to onehour. The extraction can be carried out in one or more stages and eitherbatch- Wise or continuously. In batchwise refining the treatmenttemperature is preferably approximately 25 C., and in continuousrefining preferably approximately 60 C. After the extraction withconcentrated sulfuric acid or oleum, the acid sludge is removed from theoil. The oil may then be neutralized by treatment with an alkalinematerial, such as an aqueous or alcoholic solution of NaOH, which may befol-lowed by washing with water and/ or alcohol. Alternatively, analkaline material, such as lime, may be added to the solid adsorbentwith which the oil is finished.

According to a particular aspect of the invention, the oil of type B isprepared by extracting an oil With a selec tive solvent for the aromaticcomponents of the oil fol-' lowed by subjecting the rafiinate thusobtained to a treat ment with concentrated sulfuric acid or oleum and afinish ing treatment with a solid adsorbent. This is illustrated by thefollowing example.

EXAMPLE II An oil of type B was prepared from a portion of the samestarting material as used in Example I. This portion of the startingmaterial was extracted with liquid sulfur dioxide, so as to obtain arafiinate having an aromatic content of 9% by weight. This rafiinate wastreated with oleum, neutralized and finished with Filtrol as in Run D ofExample I.

Twenty parts by weight of this oil (type B) were mixed l as describedabove in connection with the preparation of the oil of type A,extracting the hydrogenated oil with a selective solvent for thearomatic components of the oil and subjecting the ratfinate thusobtained to a finishing treatment with a solid adsorbent. Particularly,an insulating oil may be prepared as follows. The starting material isselectively hydrogenated and the hydrogenated oil is divided into twoparts. One part is treated with a solid adsorbent so as to obtain an oilof type A. The other part is extracted with a solvent selective toaromatics and the raffinate thus obtained is treated with a solidadsorbent so as to obtain an oil of type B. Finally the oils of type Aand B are mixed. Instead of treating the two oils separately with asolid adsorbent, theymay also be mixed before being treated with a solidadsorbent and the mixture treated with a solid adsorbent. Preferably aproportion of at least but not more than about 60% by weight of thehydrogenated oil is extracted. The conditions of extraction arepreferably such as to give a raffinate yield of at least 70% by weightof the extracted portion of the hydrogenated oil. The degree ofextraction of a 60% proportion of hydrogenated oil would thus be about18% of the whole hydrogenated oil. In preparing oils of type B, Whetherfrom the hydrodesulfurized or untreated starting material, the degree ofextraction should be limited to not more than by weight of the startingmaterial. The raffinate and unextracted hydrogenated oil after blendingtherefore comprises at least 80% by weight of the original hydrogenatedoil. The following example illustrates this embodiment of the invention.

EXAMPLE III The same starting material as was used in Example I wasselectively hydrogenated and steam stripped as described in Example I.Several portions of the hydrogenated oil thus obtained were extractedwith liquid sulfur dioxide under the conditions mentioned in thefollowing table.

The rafiinates thus obtained were mixed in varying proportions with theunextracted hydrogenated oil and the mixtures were finished by treatmentwith Filtrol as described in Example I. The blending proportions as wellas the B.S.I. test results of the finished electrical insulating oilsare given in the following table.

Ratlinate 13.5.1. test Unextracted hydrogenated oil, amount, percent wt.Amount, Sludge, Acid percent Number percent valu0,mg.

wt. wt. KOH/g.

40 1 0. 7 0. 6 l0 2 0. 7 0. 7 40 3 0. 8 0. 8 40 4 0. 5 0. 5 40 5 0.7 0.820 3 O. 9 O. 6 20 4 0. 8 0. B

All these blends were strongly gas-absorbing, viz., they exhibited anegative gassing coefiicient.

If desired, small quantities (e.g. between 0.01 and 1% by Weight) ofanti-oxidants may be added to the insulating oils prepared according tothe invention. As such, alkyl phenols, such as2,6-di-tert.butyl-4-methyl phenol, are particularly suitable. In thisconnection it should be observed that the insulating oils of theinvention usually have a good inhibitor susceptibility.

We claim as our invention:

A process for the production of an oxidation stable non-gassinginsulating oil from a sulfur-containing mineral lubricating oil mixturewhich also contains aromatics and boils within the range from about 300to about 500 C. consisting essentially of the steps of (l) selectivelyhydrodesulfurizing said mixture without appreciable conversion ofhydrocarbons therein to reduce the sulfur content by at least but by notmore than about by weight, (2) separating from the hydrodesulfurizedmixture by means of fractional distillation a light lubrieating oilfraction boiling from about 300 to about 500 C. having a viscosity at 20C. of from about 1 to about 9 degrees Engler and having a Pensky-Martensclosed cup flash point of at least C., (3) separating from the lightlubricating oil fraction a portion comprising at least 10% but not morethan 60% by volume thereof, (4) extracting the separated portion oflight lubricating oil fraction by means of an aromatics-selectivesolvent to a depth such that the aromatics-containing extract does notexceed 20% by volume of the total light lubricating oil References Citedin the file of this patent UNITED STATES PATENTS Van Loom et al Dec. 23,1958 Thompson July 5, 1960

